Solid state imaging apparatus, and video system using such solid state imaging apparatus

ABSTRACT

This invention prevents an end portion of the LOCOS region having a large number of defects of an MOS sensor from depletion and thereby reduces the leak current that occurs in the defects in the end portion of the LOCOS region. An n-type layer region is formed in a surface area of a p-type substrate for constituting a photodiode with the p-type layer. A LOCOS region is formed on a p + -type layer in a surface area of the silicon substrate as device separation region by oxidizing part of the silicon substrate. The n-type layer region and the LOCOS region are separated from each other by a predetermined distance. A contact region is formed and separated from the n-type layer region by a distance equal to the size of the gate electrode of the read-out transistor of the MOS sensor. A wiring layer is connected to the contact region. Then, a planarizing layer is formed to cover the n-type layer region, the LOCOS region, the gate electrode and the wiring layer.

BACKGROUND OF THE INVENTION

This invention relates to an amplifier type solid state imagingapparatus formed on a silicon substrate and, more particularly, itrelates to a solid state imaging device wherein each unit cell has aphotodiode processed for device separation by means of a silicon oxidefilm formed by oxidizing the silicon substrate. The present inventionalso relates to a solid state imaging apparatus wherein thesemiconductor substrate of the apparatus has for each unit cell a regionlocated at a position deeper than the depletion layer region operatingas a photodiode, in which region the impurity concentration of thesemiconductor slowly increases as a function of the depth in thesubstrate and part of the signal charges generated in the semiconductorsubstrate are collected by a signal storage to provide a high dynamicrange. The present invention further provides a method of manufacturingsuch a solid state imaging apparatus and a video system realized byusing such a solid state imaging apparatus.

Solid state imaging apparatuses comprising an amplifier type sensor havebeen developed in recent years. Such apparatuses are featured bydetecting optical signals by means of a photoelectric converter/storageand amplifying them in the vicinity of the photoelectricconverter/storage.

An amplifier type MOS sensor typically comprises in each unit pixels orunit cell thereof a photodiode and amplifying means including anamplifier transistor for amplifying the signal charges photoelectricallyconverted and collected by the photodiode in the silicon substrate.

FIG. 1 of the accompanying drawings schematically illustrates in crosssection part of a unit cell of a known amplifier type MOS sensor. Asseen from FIG. 1, an n-type layer region 12 that constitutes aphotodiode with a silicon substrate (p-type layer region) 10 is formedin an oxide film for device separation in a self-aligning manner. Adevice separating region 16 arranged on a p⁺-layer 14 is a silicon oxidefilm formed by oxidizing part of the silicon substrate 10, which siliconoxide film is normally referred to as LOCOS (LOCal Oxidation ofSilicon). Reference numerals 18 and 20 in FIG. 1 denote respectively acontact region and a wiring layer connected to the contact region 18,whereas reference numerals 22 and 24 denote respectively the gate of aread-out transistor and a planarizing layer.

The silicon substrate 10 is apt to become defective at and near thecorresponding end of the LOCOS region 16 due to the stress generatedduring the local oxidation. The defect, if any, by turn gives rise to anelectric current that appear as leak current of the photodiode.

Now, this problem will be discussed by referring to FIG. 2 of theaccompanying drawing.

FIG. 2 is an enlarged cross sectional view showing the boundary of thephotodiode and the LOCOS region of FIG. 1. As shown in FIG. 2, adepletion region 26 is formed around the n-type layer region 12 and adepleted region with a large number of defects (multi-defect region) 28is formed in a lower boundary area of the LOCOS region 16 locatedadjacent to the n-type layer region 12. Thus, a large number ofelectron/hole pairs will be generated by heat via the defect levels inthe silicon band gap in the multi-defect regions. Then, electrons cantransfer into the photodiode to appear as leak current of thephotodiode, which leak current can reduce the sensitivity or the S/Nratio of the solid state imaging apparatus.

Thus, since a photodiode and a photodiode are formed in a self-aligningmanner in known solid state imaging apparatus, they are accompanied bythe problem of leak current on the part of the photodiode generated dueto the defect at and near the corresponding end of the LOCOS region 16.

BRIEF SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to provide a solidstate imaging apparatus that can prevent any leak current from appearingin the photodiode and is resistive against degradation in thesensitivity, a method of manufacturing such a solid state imagingapparatus, and a video system using such a solid state imagingapparatus.

According to a first aspect of the invention, the above object isachieved by providing a solid state imaging apparatus comprising foreach unit cell a substrate of a first conductivity type, at least afirst impurity layer of a second conductivity type different from thefirst conductivity type formed in a surface area of the substrate for aphotodiode for forming a photoelectric conversion region, a deviceseparation region for the device separation of the photodiode, thedevice separation region having a second impurity layer formed in alower area thereof, and means for amplifying the signal chargescollected by the photodiode, wherein the first impurity layer isseparated from the device separation region by a predetermined distance.

According to a second aspect of the invention, there is provided a solidstate imaging apparatus comprising for each unit cell a substrate of afirst conductivity type, at least a first impurity layer of a secondconductivity type different from the first conductivity type formed in asurface area of the substrate for a photodiode for forming aphotoelectric conversion region, a device separation region for thedevice separation of the photodiode, the device separation region havinga second impurity layer formed in a lower area thereof, and means foramplifying the signal charges collected by the photodiode, wherein thefirst impurity layer is located adjacent to the second impurity layerand the apparatus further comprises for each unit cell a third impuritylayer located adjacent to the second impurity layer and formedcontinuously at least in part with the surface area of the firstimpurity region, the third impurity layer having an impurityconcentration greater than the second impurity layer.

According to a third aspect of the invention, there is provided a solidstate imaging apparatus comprising for each unit cell a substrate of afirst conductivity type, at least a first impurity layer of a secondconductivity type different from the first conductivity type formed in asurface area of the substrate for a photodiode for forming aphotoelectric conversion region, a device separation region for thedevice separation of the photodiode, the device separation region havinga second impurity layer formed in a lower area thereof, and means foramplifying the signal charges collected by the photodiode, wherein thesecond impurity layer contains a third impurity layer of the firstconductivity type located on the side of the first impurity layer andhas an impurity concentration greater than the second impurity layer andthe first impurity layer is located adjacent to the third impuritylayer.

According to a fourth aspect of the invention, there is provided amethod of manufacturing a solid state imaging apparatus comprising afirst step of forming for each unit cell a first impurity layer in asurface area of a substrate of a first conductivity type by implantingions of an impurity of a first conductivity type into the substrate toan impurity concentration level higher than the substrate, using asilicon nitride film formed on the substrate as a mask, a second step offorming a device separation region on the first impurity layer, a thirdstep of forming a second impurity layer of a second conductivity type ina surface area of the substrate and a third impurity layer of the secondconductivity type in another surface area of the substrate separatedfrom the device separation region by a predetermined distance byimplanting ions of an impurity of the second conductivity type differentfrom the first conductivity type, using an electrode formed on thesubstrate and a resist layer formed on the substrate and the deviceseparation region as a mask and a fourth step of forming a wiring layeron the second impurity layer after removing the resist layer.

According to a fifth aspect of the invention, there is provided a methodof manufacturing a solid state imaging apparatus comprising a first stepof forming for each unit cell a first impurity layer in a surface areaof a substrate of a first conductivity type by implanting ions of animpurity of a first conductivity type into the substrate to an impurityconcentration level higher than the substrate, using a silicon nitridefilm formed on the substrate as a mask, a second step of forming adevice separation region on the first impurity layer, a third step offorming a second impurity layer of a second conductivity type in asurface area of the substrate and a third impurity layer of the secondconductivity type in another surface area of the substrate adjacent tothe device separation region by implanting ions of an impurity of thesecond conductivity type different from the first conductivity type,using an electrode formed on the substrate and a resist layer formed onthe substrate and the device separation region as a mask, a fourth stepof forming a fourth impurity layer by implanting ions of the impurity ofthe first conductivity type into part of the surface area of the thirdimpurity layer adjacent to the first impurity layer to an impurityconcentration level higher than the first impurity layer, using theelectrode formed on the substrate and the resist layer formed on thesubstrate and the device separation region as a mask and a fifth step offorming a wiring layer on the second impurity layer after removing theresist layer.

According to a sixth aspect of the invention, there is provided a methodof manufacturing a solid state imaging apparatus comprising a first stepof forming for each unit cell a first impurity layer in a surface areaof a substrate of a first conductivity type by implanting ions of animpurity of a first conductivity type into the substrate to an impurityconcentration level higher than the substrate, using a silicon nitridefilm formed on the substrate as a mask, a second step of forming adevice separation region on the first impurity layer, a third step offorming a second impurity layer of a second conductivity type in asurface area of the substrate and a third impurity layer of the secondconductivity type in another surface area of the substrate adjacent tothe device separation region by implanting ions of an impurity of thesecond conductivity type different from the first conductivity type,using an electrode formed on the substrate and a resist layer formed onthe substrate and the device separation region as a mask, a fourth stepof forming a fourth impurity layer by implanting ions of the impurity ofthe first conductivity type into a surface area of the third impuritylayer to an impurity concentration level higher than the first impuritylayer, using the electrode formed on the substrate and the resist layerformed on the substrate and the device separation region as a mask and afifth step of forming a wiring layer on the second impurity layer afterremoving the resist layer.

According to a seventh aspect of the invention, there is provided amethod of manufacturing a solid state imaging apparatus comprising afirst step of forming for each unit cell a first impurity layer in asurface area of a substrate of a first conductivity type by implantingions of an impurity of a first conductivity type into the substrate toan impurity concentration level higher than the substrate, using asilicon nitride film formed on the substrate as mask, a second step offorming a device separation region on the first impurity layer, a thirdstep of forming a second impurity layer of a second conductivity type ina surface area of the substrate and a third impurity layer of the secondconductivity type in another surface area of the substrate separatedfrom the device separation region by a predetermined distance byimplanting ions of an impurity of the second conductivity type differentfrom the first conductivity type, using an electrode formed on thesubstrate and a resist layer formed on the substrate and the deviceseparation region as a mask, a fourth step of forming a fourth impuritylayer by implanting ions of the impurity of the first conductivity typeinto a surface area of the third impurity layer and a surface area ofthe substrate adjacent to the first impurity layer to an impurityconcentration level higher than the first impurity layer, using theelectrode formed on the substrate and the resist layer formed on thesubstrate and the device separation region as a mask and a fifth step offorming a wiring layer on the second impurity layer after removing theresist layer.

According to an eighth aspect of the invention, there is provided amethod of manufacturing a solid state imaging apparatus comprising afirst step of forming for each unit cell a first impurity layer in asurface area of a substrate of a first conductivity type by implantingions of an impurity of a first conductivity type into the substrate toan impurity concentration level higher than the substrate, using asilicon nitride film formed on the substrate as a mask, a second step offorming a second impurity layer by implanting ions of the impurity ofthe first conductivity type into a surface area of the substrateadjacent to the first impurity layer to a concentration level higherthan the first impurity layer, using the silicon nitride film and aresist layer formed on part of the first impurity layer as mask, a thirdstep of forming a device separation region on the first impurity layer,a fourth step of forming a third impurity layer of a second conductivitytype in a surface area of the substrate and a fourth impurity layer ofthe second conductivity type in another surface area of the substrateadjacent to the device separation region by implanting ions of animpurity of the second conductivity type different from the firstconductivity type, using an electrode formed on the substrate and aresist layer formed on the substrate and the device separation region asa mask and a fifth step of forming a wiring layer on the second impuritylayer after removing the resist layer.

According to a ninth aspect of the invention, there is provided a methodof manufacturing a solid state imaging apparatus comprising a first stepof forming for each unit cell a first impurity layer in a surface areaof a substrate of a first conductivity type by implanting ions of animpurity of a first conductivity type into the substrate to an impurityconcentration level higher than the substrate, using a silicon nitridefilm formed on the substrate as a mask, a second step of forming adevice separation region on the first impurity layer, a third step offorming a second impurity layer of a second conductivity type in asurface area of the substrate and a third impurity layer of the secondconductivity type in another surface area of the substrate separatedfrom the device separation region by a predetermined distance byimplanting ions of an impurity of the second conductivity type differentfrom the first conductivity type, using an electrode formed on thesubstrate and a resist layer formed on the substrate and the deviceseparation region as a mask, a fourth step of forming a fourth impuritylayer by implanting ions of the impurity of the first conductivity typeinto part of the surface area of the third impurity layer adjacent tothe first impurity layer to an impurity concentration level higher thanthe first impurity layer, using the electrode and the resist layerformed on the substrate and the device separation region as a mask and afifth step of forming a wiring layer on the second impurity layer afterremoving the resist layer.

According to a tenth aspect of the invention, there is provided a videosystem comprising an optical system for taking an optical image of ascene and leading the optical image to a predetermined location, animaging means comprising an array of pixels, each having at least aphotodiode region for a photoelectric conversion, a device separationregion for the device separation of the photodiode and means foramplifying the signal charges collected by the photodiode, forphotoelectrically transforming the optical image led to thepredetermined location into an electric signal representing the quantityof light of the optical image on a pixel by pixel basis and signalprocessing means for storing the electric signal produced by thephotoelectric transformation by the imaging means, wherein the imagingmeans comprises for each cell a device separation region for the deviceseparation of the photodiode, the device separation region beingprovided with a first impurity layer of a first conductivity type formedin a lower area thereof and a second impurity layer of a secondconductivity type different from the first conductivity type formed in asurface area of the substrate of the first conductivity type separatedfrom the device separation region by a predetermined distance.

According to an eleventh aspect of the invention, there is provided avideo system comprising an optical system for taking an optical image ofa scene and leading the optical image to a predetermined location, animaging means comprising an array of pixels, each having at least aphotodiode region for a photoelectric conversion, a device separationregion for the device separation of the photodiode and means foramplifying the signal charges collected by the photodiode, forphotoelectrically transforming the optical image led to thepredetermined location into an electric signal representing the quantityof light of the optical image on a pixel by pixel basis and signalprocessing means for storing the electric signal produced by thephotoelectric transformation by the imaging means, wherein the imagingmeans comprises for each cell a device separation region for the deviceseparation of the photodiode, the device separation region beingprovided with a first impurity layer of a first conductivity type formedin a lower area thereof, a second impurity layer of a secondconductivity type different from the first conductivity type formed in asurface area of the substrate of the first conductivity type and a thirdimpurity layer of the first conductivity type at least in part of thesurface area of the second impurity layer and having an impurityconcentration greater higher than the first impurity layer.

According to a twelfth aspect of the invention, there is provided avideo system comprising an optical system for taking an optical image ofa scene and leading the optical image to a predetermined location, animaging means comprising an array of pixels, each having at least aphotodiode region for a photoelectric conversion, a device separationregion for the device separation of the photodiode and means foramplifying the signal charges collected by the photodiode, forphotoelectrically transforming the optical image led to thepredetermined location into an electric signal representing the quantityof light of the optical image on a pixel by pixel basis and signalprocessing means for storing the electric signal produced by thephotoelectric transformation by the imaging means, wherein the imagingmeans comprises for each cell a device separation region for the deviceseparation of the photodiode, the device separation region beingprovided with a first impurity layer of a first conductivity type havingan impurity concentration higher than the first impurity layer formed ina lower area thereof and a third impurity layer of a second conductivitytype different from the first conductivity type formed adjacentlyrelative to the second impurity layer in a surface area of the substrateof the first conductivity type.

According to the invention, the photodiode is separated from thecorresponding end of the LOCOS region containing a large number ofdefects therein, which is made to show a high concentration level of animpurity having an conductivity type opposite to that of the photodiode.

According to the invention, the conventional completely depletedstructure of the photodiode is modified within an area where incidentlight is absorbed and attenuated but remains effective so as to arrangea completely depleted region and an undepleted signal storage regionwithin the effective stroke of incident light and provide the impurityconcentration of the undepleted semiconductor impurity layer with agradient slowly rising toward the adjacent pixel or the bottom of thesubstrate so that the signal charges generated in the undepleted regionare encouraged to transfer into the storage side due to theconcentration gradient and become distributed and only part of thesignal charges may be stored.

According to the invention, the impurity concentration of the impuritysemiconductor surrounding the photodiode is provided with a mildgradient to effectively collect the signal charges obtained by thephotoelectric conversion of incident light and discard part of the largevolume of signals generated by very bright light as waste.

According to the invention, the impurity concentration of thesemiconductor substrate with a gradient slowly rising as a function ofthe depth of the substrate in an area located below the depleted regionof the photodiode to effectively collect the signal charges obtained bythe photoelectric conversion of incident light. As a result, only partof the signal charges generated by very bright light may be stored, therest being discharged to the substrate side by diffusion to realize ahigh dynamic range.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a schematic cross sectional view of a unit cell of a knownamplifier type MOS sensor, including a photodiode.

FIG. 2 is an enlarged view showing the boundary area of the photodiodeand the LOCOS region of FIG. 1.

FIG. 3 is a schematic circuit diagram of a typical solid state imagingapparatus comprising an amplifier type MOS sensor.

FIGS. 4A through 4D are cross sectional views of a first embodiment ofsolid state imaging apparatus according to the invention and comprisingan amplifier type MOS sensor that comprises a photodiode in each unitcell, the first embodiment being shown in different manufacturing stepsin these views.

FIGS. 5A through 5E are cross sectional views of a second embodiment ofsolid state imaging apparatus according to the invention and comprisingan amplifier type MOS sensor that comprises a photodiode in each unitcell, the second embodiment being shown in different manufacturingsteps.

FIGS. 6A through 6D are cross sectional views of a third embodiment ofsolid state imaging apparatus according to the invention and comprisingan amplifier type MOS sensor that comprises a photodiode in each unitcell, the third embodiment being shown in different manufacturing steps.

FIGS. 7A through 7E are cross sectional views of a fourth embodiment ofsolid state imaging apparatus according to the invention and comprisingan amplifier type MOS sensor that comprises a photodiode in each unitcell, the fourth embodiment being shown in different manufacturingsteps.

FIGS. 8A through 8F are cross sectional views of a fifth embodiment ofsolid state imaging apparatus according to the invention and comprisingan amplifier type MOS sensor that comprises a photodiode in each unitcell, the fifth embodiment being shown in different manufacturing steps.

FIGS. 9A through 9E are cross sectional views of a sixth embodiment ofsolid state imaging apparatus according to the invention and comprisingan amplifier type MOS sensor that comprises a photodiode in each unitcell, the sixth embodiment being shown in different manufacturing steps.

FIG. 10 is a graph schematically illustrating the relationship betweenthe depth from the substrate surface and the impurity concentration ofthe photodiode of a typical solid state imaging apparatus.

FIG. 11 is a graph schematically illustrating the relationship betweenthe depth from the substrate surface and the electric potential of thephotodiode of a typical solid state imaging apparatus.

FIG. 12 is a graph schematically illustrating the relationship betweenthe depth from the substrate surface and the impurity concentration ofthe photodiode of another typical solid state imaging apparatus.

FIG. 13 is a graph schematically illustrating the relationship betweenthe depth from the substrate surface and the electric potential of thephotodiode of a still another typical solid state imaging apparatus.

FIG. 14 is a schematic cross sectional view of a seventh embodiment ofthe invention, showing the layered structure of the substrate of thesolid state imaging apparatus.

FIG. 15 is a graph showing the distribution of the impurityconcentration observed in the seventh embodiment immediately afterintroducing an impurity and also the distribution of the impurity afterthe heat treatment.

FIG. 16 is a graph schematically illustrating the relationship betweenthe depth from the substrate surface and the impurity concentration ofthe photodiode of the seventh embodiment.

FIG. 17 is a graph schematically illustrating the relationship betweenthe depth from the substrate surface and the electric potential of thephotodiode of the seventh embodiment.

FIG. 18 is a schematic cross sectional view of an eighth embodiment ofthe invention, showing the layered structure which the apparatus haswhen highly accelerated ions are implanted.

FIG. 19 is a schematic cross sectional view of the eighth embodiment ofthe invention, showing the layered structure which the apparatus hasafter highly accelerated ions have been implanted.

FIG. 20A is a schematic cross sectional view of a ninth embodiment ofthe invention, showing the layered structure of the substrate of thesolid state imaging apparatus. FIG. 20B is a graph showing thedistribution of the impurity concentration in the substrate of FIG. 20A.

FIG. 21A is a schematic cross sectional view of a tenth embodiment ofthe invention, showing the layered structure of the substrate of thesolid state imaging apparatus. FIG. 21B is a graph showing thedistribution of the impurity concentration in the substrate of FIG. 21A.

FIG. 22 is a schematic circuit diagram of a cell having a structureadapted for discharging reset charges to the substrate side.

FIGS. 23A through 23C are graphs showing the electric potential of thedetecting section of FIG. 22.

FIG. 24 is a schematic block diagram of an eleventh embodiment of theinvention that is a typical picture reading system comprising anamplifier type MOS sensor according to the invention.

FIG. 25 is a schematic block diagram of a still camera comprising a MOSsensor according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Now, the present invention will be described in greater detail byreferring to the accompanying drawing that illustrates preferredembodiments of the invention.

Throughout the drawing, the same or similar components are denotedrespectively by the same reference numerals and will not be describedrepeatedly.

FIG. 3 is a schematic circuit diagram of a typical solid state imagingapparatus comprising an amplifier type MOS sensor.

Referring to FIG. 3, the amplifier type MOS sensor is connected to avertical shift register 32 and a horizontal shift register 34 andcomprises in each unit pixel or unit cell at least a photodiode 36 andamplifying means including an amplifier transistor 38 for amplifying thesignal charges photoelectrically converted and collected in the siliconsubstrate of the sensor.

FIGS. 4A through 4D are cross sectional views of a first embodiment ofsolid state imaging apparatus according to the invention and comprisingan amplifier type MOS sensor that comprises a photodiode in each unitcell, the first embodiment being shown in different manufacturing stepsin these views.

Referring to FIG. 4A, an oxide film 44 and a silicon nitride film 46 areformed on a silicon substrate (p-type layer region) 42. Then, ions of animpurity substance are implanted into the silicon substrate 42, usingthe silicon nitride film 46 as a mask to produce a p⁺-layer 48 having animpurity concentration greater than the silicon substrate 42 on asurface area of the substrate 42.

Then, as shown in FIG. 4B, a LOCOS region 50 is formed as deviceseparating region on the p⁺-layer 48 by oxidizing part of the siliconsubstrate 42.

Thereafter, as shown in FIG. 4C, a gate electrode 52 of read-outtransistor is formed on the substrate 42. Subsequently, resist isapplied to the surface of the substrate 42 to produce a resist layer 56except the area for forming an n-type layer region 54. Note that theresist layer 56 is positionally so arranged that the space separatingthe oppositely disposed ends of the LOCOS region 50 and the n-type layerregion 54 shows a predetermined distance L as seen from FIG. 4C. Then,an n-type impurity is ion-implanted into the substrate 42, by using thegate electrode 52 and the resist layer 56 as a mask. The impurity isdiffused by heat treatment after the exfoliation of the resist layer 56,forming an n-type layer region 54 on the surface of the substrate 42.The substrate 42 and the region 54 constitute a photodiode. Then, ann-type contact region 58 is formed in a similar manner.

Note that the n-type layer region 54 and the contact region 58 areseparated from each other by a distance equal to the size of the gateelectrode 52 of the read-out transistor.

Thereafter, as shown in FIG. 4D, a wiring layer 60 is formed andconnected to the contact region 58 and, at the same time, a planarizinglayer 62 is formed on the substrate 42.

Thus, this embodiment differs from any comparable known devices in thatthe n-type layer region 54 constituting a photodiode is not formed inthe LOCOS region in a self-aligning manner. With the above describedconventional practice, a photodiode is formed in the LOCOS region in aself-aligning manner by implanting ions of an n-type impurity substance,using the LOCOS region itself as mask. With this embodiment, on theother hand, a resist layer 56 is formed in an area covering the LOCOSregion 50 and a photodiode is produced by implanting ions of an n-typeimpurity substance.

Therefore, the n-type layer region 54 of the photodiode can be separatedby a predetermined distance of L from the LOCOS region 50 in terms ofthe oppositely disposed ends thereof so that the multi-defect regionproduced at the end of the LOCOS region 50 is protected againstdepletion and hence any leak current due to the defect at the end of theLOCOS region 50 can be effectively prevented from occurring.

Note that the leak current that can appear at the end of the LOCOSregion 50 can be remarkably reduced when the distance L separating itfrom the corresponding end of the photodiode (n-type layer region) 54 isgreater than 0.1 μm.

Additionally, it has been found that the defects D in the multi-defectregion produced at the end of the LOCOS region 50 extend downwardly witha certain angle (about 59°) from the end of the LOCOS region 50. Thus,if the angle with which defects D extend from the surface of thesubstrate 42 is θ and the distance from the end of the LOCOS region 50to the corresponding end of the n-type layer region 54 is L, while thedepth of the n-type layer region 54 from the surface of substrate 42 isY, the distances L and Y may be selected to satisfy the followingformula.

Y<L tan θ

In other words, the multi-defect region can be protected againstdepletion and a leak current due to the defect at the end of the LOCOSregion 50 can be prevented from occurring by selecting the distance Lseparating the LOCOS region 50 and the n-type layer region 54 and thedepth Y of the n-type layer region 54 from the surface of the substrateso as to meet the above relationship.

Now, a second embodiment of the invention will be described.

FIGS. 5A through 5E are cross sectional views of a second embodiment ofsolid state imaging apparatus according to the invention and comprisingan amplifier type MOS sensor that comprises a photodiode in each unitcell, the second embodiment being shown in different manufacturing stepsin these views.

This embodiment differs from the above described first embodiment inthat the n-type layer region 54 of the photodiode and the LOCOS region50 of the embodiment are not separated from each other but a p⁺⁺-typedefect shielding layer is formed on the n-type layer region 54 of thephotodiode and arranged in contact with the end of the LOCOS region 50,the p⁺⁺-type layer containing a p-type impurity substance to an enhancedconcentration.

The manufacturing steps shown in FIGS. 5A and 5B are identical withthose of FIGS. 4A and 4B and hence will not be described here anyfurther.

Referring to FIG. 5C, after forming a gate electrode 52 of read-outtransistor on the substrate 42, resist is applied to the surface of thesubstrate to form a resist layer 55. Then, ions of an n-type impuritysubstance are implanted into the substrate, using the gate electrode 52and the resist layer 55 as mask. After the exfoliation of the resistlayer 55, the device is heat treated to diffuse the implanted ions inorder to produce an n-type layer region 54 on the surface of thesubstrate 42. An n-type contact region 58 is formed in a similar manner.

Then, as shown in 5D, a resist layer 56 is formed by applying resist tothe surface of the substrate except the area for forming a defectshielding layer 64 that is a p⁺⁺-type layer having an impurityconcentration level higher than the p⁺-type layer 48. Under thiscondition, ions of a p-type impurity substance are implanted into thesubstrate, using the resist layer 56 and the LOCOS region 50 as mask andthen a p⁺⁺-type defect shielding layer 64 is produced in a self-aligningmanner when the device is subjected to heat treatment.

Thereafter, as shown in FIG. 5E, a wiring layer 60 is formed andconnected to the contact region 58 and, at the same time, a planarizinglayer 62 is formed on the substrate 42.

In this second embodiment, the multi-defect region at the end of theLOCOS region 50 is protected against depletion and a leak current due tothe defect at the end of the LOCOS region 50 is prevented from occurringby forming a p⁺⁺-type defect shielding layer having an enhanced impurityconcentration level at the end of the LOCOS region 50.

FIGS. 6A through 6D are cross sectional views of a third embodiment ofsolid state imaging apparatus according to the invention and comprisingan amplifier type MOS sensor that comprises a photodiode in each unitcell, the third embodiment being shown in different manufacturing stepsin these views.

This embodiment differs from the above described second embodiment inthat a p⁺⁺-type defect shielding layer is formed to cover the entiresurface of the n-type layer region 17 of the photodiode.

The manufacturing steps shown in FIGS. 6A and 6B are identical withthose of FIGS. 4A and 4B and hence will not be described here anyfurther.

As in the case of FIG. 5C of the second embodiment, after forming a gateelectrode 52 of read-out transistor, resist is applied to the surface ofthe substrate to produce a resist layer 56.

Then, as shown in FIG. 6C, ions of an n-type impurity substance andthose of a p-type impurity substance are implanted, using the gateelectrode 52, the resist layer 56 and part of the LOCOS region 50. Afterthe exfoliation of the resist layer 56, the implanted ions are diffusedinto the substrate by heat treatment to produce an n-type layer region56 and a p⁺⁺-type defect shielding layer in a surface area of thesubstrate 42. Then, an n-type contact region 58 is formed in a similarmanner.

For the purpose of the invention, ions of the n-type impurity substancemay be implanted and heat treated before implanting and heat treatingthose of the p-type impurity substance or vice versa, although implantedions have to be accelerated in such a controlled manner that an n-typelayer region 54 is formed under a p⁺⁺-type defect shielding layer 66.

Then, as shown in FIG. 6D, a wiring layer 60 is formed and connected tothe contact region 58 and, at the same time, a planarizing layer 62 isformed on the substrate 42.

In this embodiment, a defect shielding layer 66 is formed on the entiresurface of the n-type layer region 54 of the photodiode to prevent anyleak current from occurring by way of the interface level of the siliconsubstrate 42 and the oxide film surface by extending the defectshielding layer 66 of the LOCOS region 50 over the entire surface of then-type layer region for the interface level.

Thus, in this third embodiment, the multi-defect region at the end ofthe LOCOS region 50 is protected against depletion and a leak currentdue to the defect at the end of the LOCOS region 50 is prevented fromoccurring by forming a p⁺⁺-type defect shielding layer having anenhanced impurity concentration level at the end of the LOCOS region 50as in the case of the above described second embodiment.

FIGS. 7A through 7E are cross sectional views of a fourth embodiment ofsolid state imaging apparatus according to the invention and comprisingan amplifier type MOS sensor that comprises a photodiode in each unitcell, the fourth embodiment being shown in different manufacturing stepsin these views.

This fourth embodiment has a configuration substantially same as theabove described third embodiment. Thus, the manufacturing steps shown inFIGS. 7A and 7B are identical with those of FIGS. 4A and 4B and hencewill not be described here any further.

Referring to FIG. 7C, after forming a gate electrode 52 of read-outtransistor on the substrate 42, resist is applied to the surface of thesubstrate to form a resist layer 55. Then, ions of an n-type impuritysubstance are implanted into the substrate, using the gate electrode 52and the resist layer 55 as mask.

Thereafter, as shown in FIG. 7D, ions of a p-type impurity substance areimplanted, using the gate electrode 52, the resist layer 56 and part ofthe LOCOS region 50. After the exfoliation of the resist layer 56, thedevice is heat treated to diffuse the implanted ions in order to producean n-type layer region 54 on the surface of the substrate 42. Thus, ap⁺⁺-type layer 68 a and a p⁺⁺-type defect shielding layer 68 b areformed to shield the interface level. Then, an n-type layer region 54 isformed under the p⁺⁺-type layer 68 a and the p⁺⁺-type defect shieldinglayer 68 b. An n-type contact region 58 is formed in a similar manner.

Note that the n-type layer region 54 and the LOCOS region 50 areseparated by a predetermined distance. The heat treatment after theimpurity ion implantation may be conducted not separately butsimultaneously for the n-type layer region 54, the p⁺⁺-type layer 68 aand the p⁺⁺-type defect shielding layer 68 b.

Then, as shown in FIG. 7E, a wiring layer 60 is formed and connected tothe contact region 58 and, at the same time, a planarizing layer 62 isformed on the substrate 42.

Thus, this fourth embodiment differs from the third embodiment in that,firstly, the p⁺⁺-type layer 68 a of the photodiode is separated from theend of the LOCOS region 50 as in the case of the first embodiment and,secondly, a defect shielding layer 68 b is formed independent of thep⁺⁺-type layer 68 a for shielding the interface level. The p⁺⁺-typelayer 68 b located adjacent to the LOCOS region 50 is preferably deeperthan the p⁺⁺-type layer 68 a, although the two p⁺⁺-type layers mayalternatively have a same depth.

The ion implanting and heat treating operation may be conductedsimultaneously or sequentially for the p⁺⁺-type layer 68 a and for thep⁺⁺-type layer 68 b.

In this fourth embodiment, the multi-defect region at the end of theLOCOS region 50 is protected against depletion and a leak current due tothe defect at the end of the LOCOS region 50 is prevented from occurringby forming a p⁺⁺-type defect shielding layer having an enhanced impurityconcentration level at the end of the LOCOS region 50 and separating then-type layer region 54 of the photodiode from the end of the LOCOSregion 50.

FIGS. 8A through 8F are cross sectional views of a fifth embodiment ofsolid state imaging apparatus according to the invention and comprisingan amplifier type MOS sensor that comprises a photodiode in each unitcell, the fifth embodiment being shown in different manufacturing stepsin these views. This fifth embodiment has a configuration substantiallysame as the above described second embodiment.

Referring to FIG. 8A, an oxide film 44 and a silicon nitride film 46 areformed on a p-type silicon substrate 42. Then, ions of an impuritysubstance are implanted into the silicon substrate 42, using the siliconnitride film 46 as a mask to produce a p⁺-layer 48 having an impurityconcentration greater than the silicon substrate 42 on a surface area ofthe substrate 42.

Then, as shown in FIG. 8B, resist is applied to the surface of thep⁺-type layer 48 and the silicon nitride film 46 to produce a resistlayer 70. Then, impurity ions are implanted, using the silicon nitridefilm 46 and the resist layer 70 as mask and subsequently, the resistlayer 70 is exfoliated.

Then, as shown in FIG. 8C the device is heat treated to produce ap⁺⁺-type defect shielding layer 72 adjacent to the p⁺-type layer 48.

Thereafter, a LOCOS region 50 is formed as device separating region onthe p⁺-layer 48 and the p⁺⁺-type defect shielding layer 72 by oxidizingpart of the silicon substrate 42. Then, as in the case of FIG. 4Cillustrating the first embodiment, a gate electrode 52 of read-outtransistor is formed on the substrate 42. Subsequently, resist isapplied to the surface of the substrate 42 to produce a resist layer 56.

Then, as shown in FIG. 8E, ions of an n-type impurity substance areimplanted, using the gate electrode 52, the resist layer 56 and theLOCOS region 50 as mask. After the exfoliation of the resist layer 55,the device is heat treated to diffuse the implanted ions in order toproduce an n-type layer region 54 on the surface of the substrate 42. Ann-type contact region 58 is formed in a similar manner.

Thereafter, as shown in FIG. 8F, a wiring layer 60 is formed andconnected to the contact region 58 and, at the same time, a planarizinglayer 62 is formed on the substrate 42.

Thus, while a p⁺⁺-type defect shielding layer 64 is formed outside theLOCOS region 50 at the end of the latter in the second embodiment, acorresponding p⁺⁺-type defect shielding layer 72 is formed under theLOCOS region 50 at an end portion of the latter.

Normally, a silicon nitride film 46 is firstly formed in an area outsidethat of the LOCOS region 50 and then the LOCOS region 50 is formed byoxidizing the silicon substrate 42 with oxygen or H₂O at hightemperature, using the silicon nitride film 46 as mask. It operates asdevice separating region and a device separating layer that is a p⁺-typelayer is formed by implanting ions of a p-type impurity substance, usingthe silicon nitride film 46 as a mask, before oxidizing the siliconsubstrate 42.

However, in this fifth embodiment, the p⁺⁺-type defect shielding layer72 is formed by implanting ions of a p-type impurity substance, usingthe silicon nitride film 46 as mask, before oxidizing the siliconsubstrate 42 as in the case of forming the p⁺-type device separatinglayer except that, unlike forming the p⁺-type device separating layer72, part of the area for forming the LOCOS region other than a givenportion thereof located adjacent to the photodiode is covered not by thesilicon nitride film but by the resist layer 70 so that consequently, aLOCOS region having a profile as shown in FIG. 8D is formed byimplanting ions of the p-type impurity substance, using both the resistlayer 70 and the silicon nitride film 46 as mask.

Again, the multi-defect region at the end of the LOCOS region 50 isprotected against depletion and a leak current due to the defect at theend of the LOCOS region 50 is prevented from occurring because of thep⁺⁺-type layer containing the impurity substance to an enhancedconcentration and arranged at the end of the LOCOS region.

It will be appreciated that the concentration of the p-type impuritysubstance of the p⁺-type device separating layer is not raised in asimple manner to prevent a leak current of the photodiode from flowingthrough the LOCOS region of this fifth embodiment because, if theconcentration of the p-type impurity substance of the p⁺-type deviceseparating layer is raised in a simple manner, areas other than thephotodiode including the n-type layer region 54 shown in FIG. 8F can beeroded by the impurity substance diffused from the p⁺⁺-type layer thatcontains the substance at a high concentration level and, in an extremecase, the p-type layer can reach the contact region 58 to give rise to aleak current there. Therefore, a p⁺⁺-type defect shielding layer 72 ispreferably formed in an area directly adjacent to the photodiode as inthe case of this fifth embodiment.

Now, a sixth embodiment of the invention will be described.

FIGS. 9A through 9E are cross sectional views of a sixth embodiment ofsolid state imaging apparatus according to the invention and comprisingan amplifier type MOS sensor that comprises a photodiode in each unitcell, this embodiment being shown in different manufacturing steps inthese views.

This embodiment differs from the second embodiment in that the n-typelayer region 54 and the LOCOS region 50 are separated from each other bya predetermined distance.

The manufacturing steps shown in FIGS. 9A and 9B are identical withthose of FIGS. 4A and 4B and hence will not be described here anyfurther.

Referring to FIG. 9C, after forming a gate electrode 52 of read-outtransistor on the substrate 42, resist is applied to the surface of thesubstrate to form a resist layer 56. Then, ions of an n-type impuritysubstance are implanted into the substrate, using the gate electrode 52and the resist layer 56 as mask. After the exfoliation of the resistlayer 56, the device is heat treated to diffuse the implanted ions inorder to produce an n-type layer region 54 on the surface of thesubstrate 42. An n-type contact region 58 is formed in a similar manner.

Then, as shown in 9D, a resist layer 76 is formed by applying resist tothe surface of the substrate except the area for forming a defectshielding layer 74 that is a p⁺⁺-type layer having an impurityconcentration level higher than the p⁺-type layer 48. Under thiscondition, ions of a p-type impurity substance are implanted into thesubstrate, using the resist layer 76 and part of the LOCOS region 50 asmask and then a p⁺⁺-type defect shielding layer 74 is produced in aself-aligning manner when the device is subjected to heat treatment.

Thereafter, as shown in FIG. 9E, a wiring layer 60 is formed andconnected to the contact region 58 and, at the same time, a planarizinglayer 62 is formed on the substrate 42.

In this sixth embodiment, the multi-defect region at the end of theLOCOS region 50 is protected against depletion by forming a p⁺⁺-typedefect shielding layer having an enhanced impurity concentration levelat the end of the LOCOS region 50 and additionally by separating then-type layer region 54 of the photodiode and the end of the LOCOS regionby a predetermined distance. Thus, a leak current due to the defect atthe end of the LOCOS region 50 is prevented from occurring.

The photodiode of a solid state imaging apparatus comprises a firstimpurity semiconductor layer (p⁺-type layer) for forming a chargestorage layer region and a second impurity semiconductor layer (n⁺-typelayer) having a conductivity type different from the first impuritysemiconductor layer. If the photodiode is of a type structured todischarge saturation signals to the substrate side, it additionallycomprises a third impurity semiconductor layer (p⁺-type layer) having aconductivity type same as the first impurity semiconductor layer andarranged under the second impurity semiconductor layer (see FIG. 10).

Light that enters the substrate is subjected to photoelectric conversionprincipally in the depletion layer region located between the first andsecond impurity semiconductor layers or the depletion layer regionlocated between the second and third impurity semiconductor layers.

Because of the potential gradient existing in the depletion layer, thesignal charges between the first and second impurity semiconductorlayers are collected and stored in the photodiode and, when the overallsignal charge stored in the photodiode exceeds a saturation signal levelthat is determined as a function of the barrier of the second impuritysemiconductor layer, they are discharged to the substrate side andremoved.

With photodiodes having such a structure, signal charges are principallygenerated in the depletion layer region within the substrate and all thesignal charges generated in an upper region of the signal barrier layerformed by the second impurity semiconductor layer are stored in thephotodiode. When the overall signal charge exceeds a saturation level,all the excessive signals are discharged to the substrate above thebarrier layer (see FIG. 11). Therefore, when very bright light strikesthe substrate, the signal charges stored in the storage section reachthe saturation level and would not provide output signals in response toincident light.

With photodiodes having a structure other than the above described one,including those having an adjacent overflow drain structure, the secondimpurity semiconductor layer that is a lower layer of the photodiodeshows an impurity distribution pattern defined by the impurityintroduced through the substrate surface and diffused by heat. In otherwords, the impurity in the second semiconductor layer that is a lowerlayer of the photodiode shows an impurity concentration distributionpattern that is substantially flat or mildly lowered toward the insideof the substrate (see FIG. 12).

With photodiodes having such a structure, the signals that are subjectedto photoelectric conversion in the first impurity semiconductor layerlocated in a lower area of the photodiode are apt to be discharged intothe substrate according to the gradient of impurity concentration (seeFIG. 13). Therefore, most of the signals are removed without beingdistributed to the signal charge storage section.

Thus, there is a need for a solid state imaging apparatus having astructure that can effectively suppress the loss of signals flowing intoadjacent pixels, cause the signal charges photoelectrically converted inthe substrate to be stored in the storage section of the photodiode andthe signal charges generated in the undepleted impurity semiconductorregion in a lower area of the photodiode to be diffused and partlydistributed to and stored in the storage section of the photodiode andalso make what are few signal charges to be effectively led to thestorage section for lower light and part of the large signal charges tobe stored in the storage section by diffusing the rest of the signalcharges for very bright light in order to realize a high dynamic range.

The embodiment of solid state imaging apparatus as described below isdesigned to realize a high dynamic range.

A seventh embodiment of the invention will now be described by referringto FIGS. 14 through 17.

Ions of an impurity substance are implanted in a semiconductor substratethat constitutes a photodiode down to a predetermined depth for forminga signal barrier layer by means of a highly accelerated ion implantationtechnique.

The broken line in the graph of FIG. 15 indicates the impuritydistribution of the substrate immediately after implanting ions of animpurity substance. As the substrate into which the impurity substancehas been introduced is heat treated, the impurity substance for forminga barrier layer comes to show a low distribution profile that graduallyfalls toward the signal storage section as indicated by the solid linein the graph of FIG. 15. Then, the impurity concentration of thesubstrate will show a distribution profile as shown in FIG. 16 when animpurity region is formed as signal storage region in the substrate.

Referring to FIG. 14, a LOCOS layer 84 is formed as device separatinglayer on the surface A of the substrate 80 prior to forming a signalcharge barrier layer 82 in the substrate 80. The LOCOS layer 84 has athickness that gradually increases from the peripheral area toward thecenter of device separation.

After forming the LOCOS layer 84, ions of an impurity substance areimplanted by means of a highly accelerated ion implantation technique 86to produce a signal charge barrier layer deep in the substrate. Then,the LOCOS layer 84 operates as mask for decelerating accelerated ions sothat the barrier layer 82 is formed at a relatively shallow level belowthe LOCOS layer 84.

Referring to FIG. 15 illustrating the distribution of the impurityconcentration, an interdevice barrier layer 82 is formed below the LOCOSlayer 84 so that signal charges are apt to be stored in the storagesection and signals can be prevented from being mixed with those ofadjacent pixels.

In a photodiode having an impurity concentration distribution profile asdescribed above by referring to FIG. 16, some of the signal charges thatare photoelectrically converted by incident light h ν in regions underthe depletion layer are distributed from depth A′ as viewed from thesurface A of the substrate further down to the inside A″ of thesubstrate, while the others are distributed to the signal storagesection.

Thus, in a photodiode having an impurity concentration distributionprofile as shown in FIG. 16, signal charges are apt to transfer to thestorage section when they are few in number, whereas they show adownward gradient toward the substrate when the substrate is irradiatedwith very bright light so that the storage section is less liable to besaturated with signals as excessive signal charges are diffused towardthe substrate to increase the dynamic range where the number of storedsignals is increased in response to the intensity of incident light.

While the LOCOS layer is used as mask for deceleration in the abovedescribed seventh embodiment, any other mask may alternatively be usedfor the purpose of the invention.

Now, an eighth embodiment of the invention will be described byreferring to FIG. 18.

For the embodiment of FIG. 18, scantily accelerated impurity ions areimplanted selectively in an interdevice region 82 defined by aphotoresist mask 90 in order to form a barrier layer in a relativelyshallow area. Reference numeral 92 denotes ions implanted forinterdevice separation.

Thereafter, semiconductor layers 94 and 96 and a source/drainsemiconductor activation region 98 are formed under the surface of theinterdevice region 82, thereby forming a photodiode, as is illustratedin FIG. 19. A pair of transistor gates 100 and 102 are formed on thesurface of the substrate 80.

The above arrangement gives rise to an effect same as the abovedescribed sixth embodiment. Additionally, a barrier layer can bearranged most effectively in a region separating adjacent pixels byintroducing an impurity substance to a depth intermediate between thesubstrate surface and the bottom barrier layer.

Thus, with the seventh and eighth embodiments, the signal charges thathave been photoelectrically converted in a deep region of thesemiconductor substrate are partly diffused in a gentle manner andtransfer into an upper photodiode region that operates also as storagesection. On the other hand, any possible diffusion of the chargesgenerated in a lower photodiode region is suppressed by the surroundinghighly concentrated impurity region to prevent signals from flowing intoadjacent pixels to become lost.

In the current attempt for down-sizing the cells of amplifier type solidstate imaging apparatus, efforts are being made to reduce the number ofMOS transistors, although the existence of a read-out transistor canfrustrate the attempt of down-sizing. Alternatively, the degradation ofthe sensitivity of a photodiode may be prevented by eliminating the useof a read-out transistor.

A ninth embodiment of the invention is configured to eliminate the useof a read-out transistor. This embodiment is realized by carrying out awell diffusion process after the operation of highly accelerated ionimplantation.

FIG. 20A is a schematic view of the ninth embodiment of solid stateimaging apparatus according to the invention, showing its layeredstructure, and FIG. 20B is a graph showing how the impurityconcentration is distributed in the substrate of the embodiment of FIG.20A.

A potential barrier is provided to prevent electrons from beingdischarged to the substrate side and avoid any possible degradation inthe sensitivity of the photodiode due to the diffusion potentialgradient of the p-well 108 on the n-type substrate 106. This potentialbarrier is formed by placing the portion of the p-well 108 having thehighest concentration at the position located deepest from the surfaceof the substrate. The depth of the position of the portion of the p-wellhaving the highest concentration is at least 2 μm from the substratesurface (see FIG. 20B). In FIG. 20A, reference numeral 110 denotes ann-type diffusion layer.

Thus, the portion of the p-well 108 having the highest concentration islocated at the position deepest from the surface of the substrate and adiffusion potential gradient that can collect photoelectrons at thesurface side of the substrate is formed from there up to the substratesurface. Therefore, the probability with which photoelectrons aredischarged to the substrate side is reduced and, to the contrary, theprobability with which photoelectrons are discharged to the photodiodeat the surface of the substrate is raised to consequently improve thesensitivity of the photodiode.

Note that photoelectric conversion of visible light is carried out in anarea with a depth less than about 2 μm in the (Si) substrate and,therefore, the portion of the p-well 108 having the highestconcentration should be placed at a position at least 2 μm deep in thesubstrate.

FIG. 21A is a schematic view of a tenth embodiment of solid stateimaging apparatus according to the invention, showing its layeredstructure, and FIG. 21B is a graph showing how the impurityconcentration is distributed in the substrate of the embodiment of FIG.2.

In this tenth embodiment, after forming an ordinary well, a potentialbarrier is formed against photoelectrons at the deepest position bymeans of MeV ion implantation. With this arrangement, the probabilitywith which photoelectrons are discharged to the n-type substrate side isreduced to improve the sensitivity of the photodiode.

This VOD structure for discharging signal charges to the substrate sidemakes the provision of a read-out transistor unnecessary. Unlike the VODstructure of a CCD, signal charges are discharged by manipulating thepotential of a detecting section by means of a cell address capacitor.

FIG. 22 is a circuit diagram that can be used for the cell sectionhaving a structure for discharging charges to the substrate side. Thiscircuit will be described by referring to FIGS. 23A through 23C, showingthe electric potential of the detecting section.

Referring to FIGS. 23A through 23C, an unit cell 114 comprises aphotodiode 116, a capacitor 118 and a transfer transistor 120. Thecapacitor 118 is connected between the gate of the transfer transistor120 and an address line Ad. Reference numeral 122 denotes a detectingsection.

The relationship among the diffusion layer 110, the p-well 108 and thesubstrate 106 is illustrated in FIGS. 23A through 23C in terms ofelectric potential, of which FIG. 23A shows the electric potentials ofthese components when no photoelectrons are introduced into thedetecting section 122 of FIG. 22. The address line Ad has to be broughtto level “L” to reset this relationship. Since the detecting section 122is reversely biased relative to the p-well 108 under this condition,electrons are swept out to the n-type substrate 106 by way of the p-well108. Thereafter, the electric potential of the address line Ad isrestored to complete a reset operation (FIG. 23C).

Since the provision of a read-out transistor is unnecessary with theabove arrangement, the sensitivity of the photodiode is improved and thecell can be further down-sized.

Note that the above described first through tenth embodiments relate toan amplifier type MOS sensor that can be used for an solid state imagingapparatus according to the invention. A MOS type solid state imagingapparatus according to the invention and comprising a MOS sensor of theabove described type is remarkable in that it requires only small powerand low voltages, shows a good S/N ratio and operates with a singlepower source. Now, various systems that can be realized by utilizing aMOS type solid state imaging apparatus according to the invention willbe described.

FIG. 24 is a schematic block diagram of an eleventh embodiment of theinvention that is a picture reading system comprising an amplifier typeMOS sensor according to the invention as an image detecting section.

Referring to FIG. 24, this video reading system comprises an opticalsystem 130, an amplifier type MOS sensor 132 and a signal applicationsection 134.

The optical system 130 operates to lead an optical image to theamplifier type MOS sensor 132 and typically comprises componentsselected from lenses, prisms, pinholes, dichroic mirrors, focusingoptical fibers, concave mirrors, convex mirrors, color filters, shuttermechanisms, iris mechanisms and other optical devices according to itsapplication.

The amplifier type MOS sensor 132 converts the optical image led in bythe optical system 130 into a set of video signals, each of whichreflects the quantity of light of the spot of the image it represents.If the video reading system involves a video camera, the signalapplication section 134 operates to transform the video signalstransmitted from the MOS sensor 132 into composite signals conforming tothe PAL system, the NTSC system or some other appropriate system.

The MOS sensor 132 can be driven to operate by means of a single powersource and typically comprises photodiodes in its light receivingsection for converting light into electric signals. Photodiodescorrespond to pixels that are arranged into a matrix. In order todown-size each pixel, each photodiode is made to occupy a small area andhence its output power is small. Thus, an amplifier (transistor) isarranged to amplify the small output of the photodiode in correspondenceto the pixel. However, the amplifier (transistor) generates noise when asignal passes therethrough (a noise component that is inevitable becauseof the intrinsic characteristic of an amplifying transistor). This noiseis canceled to single out the signal component by carrying out a seriesof operations including resetting the output of each of the photodiodesof the MOS sensor 132, retaining the output signal (noise component) ofthe amplifying transistor and canceling the retained output signal(noise component) and the output of the amplifier (transistor) before orafter the resetting operation (“signal component”+“noise component”).

The MOS sensor 132 can produce an output that is free from 1/f noise andhas an output current level of 1 μA or more with the voltage amplitudeof the output signal less than 10 mV. Additionally, the dynamic range ofthe output of this MOS sensor 132 can be improved to 70 dB or more thatcorresponds to the dynamic range of CCD sensor. With an appropriatesignal processing operation, the dynamic range can be further improvedto 90 dB that corresponds to the dynamic range of silver salt film.

As a result, various video systems can be realized by utilizing a highlysensitive amplifier type MOS sensor as an imaging device that requiresonly a single power source to achieve a reduced power consumption leveland a low voltage level and shows an excellent S/N ratio.

Now, a twelfth embodiment of the invention will be described.

This twelfth embodiment is realized by applying an amplifier type MOSsensor according to the invention to a still camera.

FIG. 25 is a schematic block diagram of a still camera comprising a MOSsensor according to the invention.

Referring to FIG. 25, the still camera 150 comprises an optical system152 typically including a lens system and an iris necessary for shootinga scene, a mirror 154 for guiding the image obtained by the opticalsystem 152, another mirror for leading the reflected light from themirror 154 to a finder 158, a MOS sensor 160 located behind the mirror154 at a position where the image picked up by the optical system isfocused, an imaging circuit 162 for reading the video signals fordifferent primary colors obtained from the MOS sensor 160, an A/Dconversion circuit 164 for converting the read signals into digitalsignals, a frame memory 166 for retaining the converted digital signalson a frame by frame basis, a compression circuit 168 for compressing theretained digital signals also on a frame by frame basis, a memory card172 for storing the video data and a card control circuit 170 forcontrolling the operation of writing the compressed video data from thecompression circuit 168 into the memory card 172.

The mirror 154 is retractably arranged on the optical path connectingthe optical system 152 and the focal plane of the MOS sensor 160. Itdistributes the image picked up by the optical system 152 to the finder158 by way of the mirror 156 when it is located on the optical path,whereas it operates as a shutter that causes the image picked up by theoptical system 152 to be focused on the focal plane of the MOS sensor160 when it is held to its retracted position.

With a still camera having the above described configuration, when ascene is shot by the camera as the shutter button (not shown) isdepressed, an image thereof is picked up by the optical system 152 anand focused in the MOS sensor 160 by way of the mirror 154. The MOSsensor 160 is a solid state imaging apparatus according to the inventionand, as the image picked up by the optical system 152 is focused, theoptical image is transformed into an electric signal representing thequantity of light of the optical image on a pixel by pixel basis. Inorder to reproduce a color image, the MOS sensor 160 is provided on thefocal plane side thereof with a color filter array for providing eachpixel with a color filter of red, green or blue.

In the imaging circuit 162, the electric signal produced from the MOSsensor 160 is divided into signals for red, green and blue before theyare transmitted to the A/D conversion circuit 164, where the electricsignals for red, green and blue coming from the imaging circuit 162 areconverted into digital signals, which are then temporarily retained inthe frame memory on a frame by frame basis.

The digital signals retained by the frame memory 166 are compressed bythe compression circuit 168 on a frame by frame basis and thentransmitted to the card control circuit 170, which card control circuit170 controls the operation of the memory card 172 for storing thecompressed video data.

Thus, each time the shutter button of the still camera is depressed, astill picture is taken by the camera and the signals representing thepicture is compressed for the frame and stored in the memory card 172.Note that the memory card 172 is removably arranged in the camera. Thepictures stored in the memory card 172 are then fed to areading/reproducing device (not shown) that decompresses the compressedimage data for each of the pictures so that the restored image may bedisplayed on the screen of a monitoring unit or the image data may besent to a hard copy producing unit such as a video printer to reproducethe image in the form of a hard copy.

Thus, this second embodiment is a highly sophisticated high performancecompact still camera that can take a number of pictures successivelywithin a second but requires only a single power source to achieve areduced power consumption level and a low voltage level and shows anexcellent S/N ratio. In other words, the fixed pattern noise componentthat used to be a serious problem of known MOS sensors can be canceledin a short period of time in this embodiment to produce high qualitypictures with a good S/N ratio.

It will be appreciated that the present invention contemplates not onlyvideo reading apparatus and still cameras as described above but alsovideo reading apparatus to be used in video cameras, facsimile machinesand other copying machines.

Additional advantages and modifications will readily occurs to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A solid state imaging device comprising: a firstsemiconductor layer of a first conductivity type; an element isolationregion formed on said first semiconductor layer to define an elementforming region; a first impurity region of a second conductivity typeformed at a surface region adjacent to said element isolation region,said first impurity region and said semiconductor layer forming aphotodiode receiving an incident light at a surface between said firstimpurity region and said first semiconductor layer; and a secondimpurity region of the first conductivity type formed in a surfaceregion of said first impurity region adjacent to said element isolationregion; wherein a channel stopper region of the first conductivity typeis formed under said element isolation region, and said channel stopperregion includes at least a high impurity portion adjacent to said firstimpurity region.
 2. A solid state imaging device comprising: a firstsemiconductor layer of a first conductivity type; an element isolationregion formed on said first semiconductor layer to define an elementforming region; a first impurity region of a second conductivity typeformed at a surface region adjacent to said element isolation region,said first impurity region and said semiconductor layer forming aphotodiode receiving an incident light at a surface between said firstimpurity region and said first semiconductor layer; a second impurityregion of the first conductivity type formed in a surface region of saidfirst impurity region adjacent to said element isolation region; and asecond semiconductor layer of the second conductivity type formedbeneath said first semiconductor layer, said second semiconductor layerbeing provided on a semiconductor substrate and having an impurityconcentration distribution pattern that is substantially flat or mildlylowered toward the inside of the semiconductor substrate; wherein saidincident light reaching at the surface between said first impurityregion and said first semiconductor layer is photoelectric-transformedat a first depletion layer formed between said first impurity regionforming a signal charge storage region and said first semiconductorlayer, and excess charges generated at the first depletion layer aredischarged into said semiconductor substrate via a second depletionlayer formed between said second semiconductor layer and saidsemiconductor substrate.
 3. A solid state imaging device comprising: afirst semiconductor layer of a first conductivity type; an elementisolation region formed on said first semiconductor layer to define anelement forming region; a first impurity region of a second conductivitytype formed at a surface region of said element forming region, thefirst impurity region being separated from said element isolation regionby a predetermined distance and said first impurity region and saidfirst semiconductor layer forming a photodiode receiving an incidentlight at a surface between said first impurity region and said firstsemiconductor layer; and a second impurity region of the firstconductivity type being so formed as to bridge the distance between saidfirst impurity region and said element isolation region at a surfaceregion of said first semiconductor layer.
 4. A solid state imagingdevice according to claim 3, wherein said second impurity region has animpurity concentration higher than that of said first impurity region.5. A solid state imaging device according to claim 3, further comprisinga third impurity region formed on said first impurity region to cover awhole surface of said first impurity region.
 6. A solid state imagingdevice according to claim 3, wherein a channel stopper region of thefirst conductivity type is formed under said element isolation region.7. A solid state imaging device according to claim 3, further comprisinga second semiconductor layer of the second conductivity type formedbeneath said first semiconductor layer, said second semiconductor layerbeing provided on a semiconductor substrate of the first conductivitytype and having an impurity concentration distribution pattern that issubstantially flat or mildly lowered toward an inside of thesemiconductor substrate; wherein said incident light reaching at thesurface between the first impurity region and said first semiconductorlayer is photoelectric-transformed at a first depletion layer formedbetween said first impurity region forming a signal charge storageregion and said first semiconductor layer, and excess charges generatedat the first depletion layer are discharged into said semiconductorsubstrate via a second depletion layer formed between said secondsemiconductor layer and said semiconductor substrate.
 8. A solid stateimaging device comprising: a semiconductor substrate of a firstconductivity type; an element isolation region formed on saidsemiconductor substrate to define an element forming region; a channelstopper region of the first conductivity type formed beneath saidelement isolation region; a first impurity region of a secondconductivity type formed at a surface region of said element formingregion adjacent to said channel stopper region, said first impurityregion and said semiconductor layer forming a photodiode receiving anincident light at a surface between said first impurity region and saidsemiconductor substrate; and a second impurity region of the firstconductivity type formed in a surface region of said first impurityregion adjacent to said element isolation region; wherein said channelstopper region is formed at a portion of the surface region in saidsemiconductor substrate shallower than that of said first impurityregion.
 9. A solid state imaging device comprising: a semiconductorsubstrate of a first conductivity type; an element isolation regionformed on said semiconductor substrate to define an element formingregion; a channel stopper region of the first conductivity type formedbeneath said element isolation region; a first impurity region of asecond conductivity type formed at a surface region of said elementforming region adjacent to said channel stopper region, said firstimpurity region and said semiconductor layer forming a photodiodereceiving an incident light at a surface between said first impurityregion and said semiconductor substrate; and a second impurity region ofthe first conductivity type formed in a surface region of said firstimpurity region adjacent to said element isolation region; wherein saidsecond impurity region has an impurity concentration higher than that ofsaid channel stopper region and said first impurity region.